1. Field of the Invention
This invention generally relates to coatings for components exposed to high temperatures, such as the hostile thermal environment of a gas turbine engine. More particularly, this invention is directed to a protective coating for a thermal barrier coating (TBC) on a gas turbine engine component, in which the protective coating has a low thermal conductivity, and may be resistant to infiltration by contaminants present in the operating environment of a gas turbine engine.
2. Description of the Related Art
Higher operating temperatures for gas turbine engines are continuously sought in order to increase their efficiency. However, as operating temperatures increase, the high temperature durability of the components within the hot gas path of the engine must correspondingly increase. Significant advances in high temperature capabilities have been achieved through the formulation of nickel and cobalt-base superalloys. Nonetheless, certain components of the turbine, combustor and augmentor sections of a gas turbine engine can be required to operate at temperatures at which the mechanical properties of such alloys are insufficient. For this reason, these components are often protected by a thermal barrier coating (TBC).
TBC's are typically formed of ceramic materials deposited by plasma spraying, flame spraying and physical vapor deposition (PVD) techniques. TBC's employed in the highest temperature regions of gas turbine engines are most often deposited by PVD, particularly electron-beam PVD (EBPVD), which yields a strain-tolerant columnar grain structure that is able to expand and contract without causing damaging stresses that lead to spallation. Similar columnar microstructures can be produced using other atomic and molecular vapor processes, such as sputtering (e.g., high and low pressure, standard or collimated plume), ion plasma deposition, and all forms of melting and evaporation deposition processes (e.g., cathodic arc, laser melting, etc.). In contrast, plasma spraying techniques such as air plasma spraying (APS) deposit TBC material in the form of molten splats, resulting in a TBC characterized by a degree of inhomogeneity and porosity.
Various ceramic materials have been proposed as TBC's, the most notable of which is zirconia (ZrO2) that is partially or fully stabilized by yttria (Y2O3) magnesia (MgO) or another alkaline-earth metal oxides, or ceria (CeO2) or another rare-earth metal oxides to yield a tetragonal microstructure that resists phase changes. Still other stabilizers have been proposed for zirconia, including hafnia (HfO2) (U.S. Pat. No. 5,643,474 to Sangeeta) and gadolinia (gadolinium oxide; Gd2O3) (U.S. Pat. No. 6,177,200 to Maloney). U.S. Pat. Nos. 5,512,382 and 5,624,721 to Strangman mention yttria-stabilized hafnia as a possible TBC material, though neither of these patents suggests what a suitable composition or microstructure might be. Still other proposed TBC materials include ceramic materials with the pyrochlore structure A2B2O7, where A is lanthanum, gadolinium or yttrium and B is zirconium, hafnium and titanium (U.S. Pat. No. 6,117,560 to Maloney). However, yttria-stabilized zirconia (YSZ) has been the most widely used TBC material. Reasons for this preference for YSZ are believed to include its high temperature capability, low thermal conductivity, and relative ease of deposition by plasma spraying, flame spraying and PVD techniques.
To protect a gas turbine engine component from its hostile thermal environment, the thermal conductivity of a TBC is of considerable importance. Lower thermal conductivities enable the use of a thinner coating, reducing the weight of the component, and/or reduce the amount of cooling airflow required for air-cooled components such as turbine blades. Though the thermal conductivity of YSZ decreases with increasing yttria content, the conventional practice has been to partially stabilize zirconia with six to eight weight percent yttria (6-8% YSZ) to promote spallation resistance. Ternary YSZ systems have been proposed to reduce the thermal conductivity of YSZ. For example, commonly-assigned U.S. Pat. No. 6,586,115 to Rigney et al. discloses a TBC of YSZ and alloyed to contain certain amounts of one or more alkaline-earth metal oxides (magnesia, calcia (CaO), strontia (SrO) and barium oxide (BaO)), rare-earth metal oxides (ceria, gadolinium oxide, lanthana (La2O3), neodymia (Nd2O3), and dysprosia (Dy2O3)), and/or such metal oxides as nickel oxide (NiO), ferric oxide (Fe2 O3), cobaltous oxide (CoO), and scandium oxide (Sc2O3). According to Rigney et al.; when present in sufficient amounts these oxides are able to significantly reduce the thermal conductivity of YSZ by increasing crystallographic defects and/or lattice strains. Another proposed ternary system based on YSZ and said to reduce thermal conductivity is taught in U.S. Pat. No. 6,025,078 to Rickerby et al. The additive oxide is gadolinium oxide, dysprosia, erbia (Er2O3), europia (Eu2O3) praseodymia (Pr2O3), urania (UO2) or ytterbia (Yb2O3), in an amount of at least five weight percent to reduce phonon thermal conductivity.
Additions of oxides to YSZ coating systems have also been proposed for purposes other than lower thermal conductivity. For example, U.S. Pat. No. 4,774,150 to Amano et al. discloses that bismuth oxide (Bi2O3), titania (TiO2), terbia (Tb4O7), europia and/or samarium oxide (Sm2O3) may be added to certain layers of a YSZ TBC for the purpose of serving as luminous activators.
To protect a gas turbine engine component from its hostile thermal environment, the thermal conductivity of a TBC is of considerable importance. Lower thermal conductivities enable the use of a thinner coating, reducing the weight of the component, and/or reduce the amount of cooling airflow required for air-cooled components such as turbine blades. Though the thermal conductivity of YSZ decreases with increasing yttria content, the conventional practice has been to partially stabilize zirconia with six to eight weight percent yttria (6-8% YSZ) to promote spallation resistance. Ternary YSZ systems have been proposed to reduce the thermal conductivity of YSZ. For example, commonly-assigned U.S. Pat. No. 6,586,115 to Rigney et al. discloses a TBC of YSZ and alloyed to contain certain amounts of one or more alkaline-earth metal oxides (magnesia, calcia (GaO), strontia (SrO) and barium oxide (BaO)), rare-earth metal oxides (ceria, gadolinium oxide, lanthana (La2O3), neodymia (Nd2O3), and dysprosia (Dy2O3)), and/or such metal oxides as nickel oxide (NiO), ferric oxide (Fe2O3), cobaltous oxide (CoO), and scandium oxide (Sc2O3). According to Rigney et al., when present in sufficient amounts these oxides are able to significantly reduce the thermal conductivity of YSZ by increasing crystallographic defects and/or lattice strains. Another proposed ternary system based on YSZ and said to reduce thermal conductivity is taught in U.S. Pat. No. 6,025,078 to Rickerby et al. The additive oxide is gadolinium oxide, dysprosia, erbia (Er2O3), europia (Eu2O3), praseodymia (Pr2O3), urania (U02) or ytterbia (Yb2O3), in an amount of at least five weight percent to reduce phonon thermal conductivity.
The service life of a TBC system is typically limited by a spallation event brought on by thermal fatigue. In addition to the CTE mismatch between a ceramic TBC and a metallic substrate, spallation can be promoted as a result of the TBC being contaminated with compounds found within a gas turbine engine during its operation. A notable example is a mixture of several different compounds, typically calcia, magnesia, alumina and silica, referred to herein as CMAS. CMAS has a relatively low melting eutectic (about 1190° C.) that when molten is able to infiltrate to the cooler subsurface regions of a TBC, where it resolidifies. During thermal cycling, the CTE mismatch between CMAS and the TBC promotes spallation, particularly TBC deposited by PVD and APS due to the ability of the molten CMAS to penetrate their columnar and porous grain structures, respectively.
It would be desirable if improved TBC materials were available that exhibited low thermal conductivities, and preferably also exhibited resistance to spallation attributable to CMAS infiltration.